Understanding and manipulating the crystallization of Sn–Pb perovskites for efficient all-perovskite tandem solar cells

All-perovskite tandem solar cells are promising as next-generation high-efficiency photovoltaic devices. However, further progress in tin-lead (Sn–Pb) mixed perovskites, which are essential as the narrow-bandgap bottom sub-cell, is hampered by unbalanced crystallization processes, leading to inhomogeneous films and reduced power conversion efficiency (PCE). Here we provide a complete understanding of the formation of Sn–Pb films, from the precursor solution to the final film. We find that the total crystallization barrier for Sn-based perovskites is limited by the desorption of dimethyl sulfoxide (DMSO), while Pb-based perovskites experience a smaller DMSO desorption barrier. By engineering the reaction barrier in mixed films via tailoring the DMSO content, we obtain synchronous Sn–Pb perovskite crystallization and high-quality homogeneous films. On the basis of this understanding, we demonstrate single-junction Sn–Pb perovskite solar cells with a PCE of 22.88% and all-perovskite tandem devices with a certified PCE of 28.87%, fabricated by antisolvent-free methods. The unencapsulated tandem devices retain 87% of their initial PCE after about 450 h with maximum power point tracking under 1 sun illumination.